The invention relates to a process for the removal of mercaptans from light hydrocarbon mixtures.
As a rule, gasolines originating from sulfur-containing petroleum contain a considerable amount of mercaptans. This holds both for gasolines obtained by atmospheric distillation of crude mineral oil and for gasolines obtained by conversion of heavy hydrocarbon oils, such as catalytic cracking, thermal cracking and hydrocracking. The presence of mercaptans in gasolines is undesirable, since they are responsible for an unpleasant odor of the gasolines and since in the combustion of the gasolines the sulphur present in the mercaptans finds its way into the atmosphere as oxides, such as e.g. sulphur dioxide. If the aim is to upgrade the gasoline by subjecting it to catalytic reforming, the sulphur present in the mercaptans is a serious drawback in connection with the poisoning of the reforming catalyst which typically contains at least one noble metal. It has already been proposed in the past to subject gasolines containing mercaptans to a chemical treatment which results in the conversion of the mercaptans into disulphides, which disulphides, like mercaptans, are soluble in the gasolines. Such a gasoline treatment only partially, solves the problem since, although the unpleasant odor of the gasoline is removed, the sulphur remains in the gasolines. Both in gasoline combustion and when gasolines are used as the feed for a catalytic reforming unit, the above-mentioned problems will be undiminished, since they are caused by the presence of sulphur in the gasolines and not by the type of compound in which the sulphur is present. For the last-mentioned problems a solution can only be found by removing the sulphur present in the mercaptans from the gasolines. This may be effected by subjecting the gasolines to a high-pressure catalytic hydrotreatment, in which sulphur is removed from the mercaptans in the gasolines in the form of hydrogen sulphide. Although such hydrotreatment can achieve an almost complete removal of the sulphur present in the mercaptans in the gasolines, this treatment has nevertheless a serious drawback. As is the case with all the high-pressure catalytic hydrotreatments, the present mercaptan removal is a costly process.
It will be clear from the above that for the removal of mercaptans from gasolines there is an urgent need for a process which, with respect to the attainable sulphur content in the product, gives results that are comparable with those of the high-pressure catalytic hydrotreatment, without being attended with the high costs of the latter treatment. Since, as a rule, also in liquefied petroleum gases a considerable amount of mercaptans is present, it would be desirable if the said process could also be used for removing mercaptans from this fuel, in view of air pollution by sulphur dioxide when using this fuel.
Applicants have carried out an investigation to find out whether the above-mentioned requirement can be satisfied by using a solid adsorbent. Since it is customary in the removal of small amounts of organic contaminants from product streams to regenerate the adsorbent loaded with contaminants by means of a treatment with an oxygen-containing gas at a temperature higher than 400.degree. C., the adsorbent concerned should additionally have a sufficient thermal stability.
In connection with this additional requirement the investigation was restricted to inorganic materials. Both amorphous and crystalline materials were used in the investigation. Amorphous alumina and amorphous silica as well as the crystalline aluminum silicates 5A, 13X and ferrierite were found to be unsuitable for the present purpose. These materials do have a sufficiently high thermal stability to make them eligible for use, but their adsorption capacity for mercaptans is too low.
Surprisingly, it has been found that certain crystalline metal silicates, a number of which have recently been synthesized, as described e.g. in U.S. Pat. No. 4,208,305 incorporated by reference, are excellently suited for use as adsorbent for the removal of mercaptans from light hydrocarbon mixtures.
The said crystalline metal silicates have not only a sufficiently high thermal stability to enable them to be regenerated without problems with the aid of a high-temperature treatment, but also a very high adsorption capacity for mercaptans. By using the said crystalline metal silicates as adsorbents, products can be prepared with a sulphur content corresponding with that obtained when using a high-pressure catalytic hydrotreatment. The said crystalline metal silicates are not only very suitable for removing mercaptans from hydrocarbon mixtures boiling in the gasoline range, but also for the removal of mercaptans from liquefied petroleum gases.
The present patent application therefore relates to a process for the removal of mercaptans from light hydrocarbon mixtures, the light hydrocarbon mixtures being contacted with certain crystalline metal silicate as hereinafter defined.